Messages in this thread | | | Date | Mon, 16 Nov 2015 13:45:16 -0500 | From | Tejun Heo <> | Subject | Re: ptrace() hangs on attempt to seize/attach stopped & frozen task |
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Hello, Oleg.
Sorry about the delay.
On Tue, Nov 10, 2015 at 09:20:17PM +0100, Oleg Nesterov wrote: > > We simply need to reimplement cgroup freezer so that its userland > > visible state is well defined (most likely jobctl stop). Right now, > > it's allowing userland to trigger "stuck somewhere in the kernel" > > condition, so interactions with frozen tasks are naturally broken. > > I agree, the freezer is not perfect, and it needs changes. > > Still I think this needs a fix in ptrace code. At least we should not > wait in TASK_UNINTERRUPTIBLE state. > > And perhaps we can simply remove this logic? I forgot why do we hide this > STOPPED -> RUNNING -> TRACED transition from the attaching thread. But the > vague feeling tells me that we discussed this before and perhaps it was me > who suggested to avoid the user-visible change when you introduced this > transition...
Heh, it was too long ago for me to remember much. :)
> Anyway, now I do not understand why do we want to hide it. Lets consider > the following "test-case", > > void test(int pid) > { > kill(pid, SIGSTOP); > waitpid(pid, NULL, WSTOPPED); > > ptrace(PTRACE_ATTACH-OR-PTRACE_SEIZE, pid, 0,0); > > assert(ptrace(PTRACE_DETACH, pid, 0,0) == 0); > } > > Yes, it will fail if we remove JOBCTL_TRAPPING. But it can equally fail > if SIGCONT comes before ATTACH, so perhaps we do not really care? > > Jan, Pedro, do you think the patch below can break gdb somehow? With this > patch you can never assume that waitpid(WNOHANG) or ptrace(WHATEVER) will > succeed right after PTRACE_ATTACH/PTRACE_SEIZE, even if you know that the > tracee was TASK_STOPPED before attach. > > Tejun, do you see any reason to keep JOBCTL_TRAPPING?
Hmmm... It's nasty tho. We're breaking a guaranteed userland behavior to mask a deficiency (IMHO it's an outright bug) in a different subsystem. The problem here is that cgroup-frozen threads become un-runnable on a running system and it doesn't make sense to me to work around that from all the affected places rather than fixing it at the source especially if that involves breaking a known supported userland behavior. This isn't different from the frozen processes failing to respond to SIGKILL. I'd be a lot more comfortable stating that cgroup freezer is currently broken rather than diddling with subtle ptrace semantics.
Thanks.
-- tejun
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